Policykliczny poziom tufitowy w miocenie okolic Gliwic

Abstract

 W profilu nadgipsowych osadów badenu okolic Gliwic występuje zespół wkładek skał piroklastycznych w formie policyklicznego poziomu tufitowego. Szczegółowe badania składu ziarnowego, mineralnego i chemicznego tych skał doprowadziły do określenia ich ryolitowego charakteru i do wyróżnienia poszczególnych etapów depozycji materiału wulkanicznego. Analiza stratygraficzna profilu osadów miocenu umożliwiła sformułowanie wniosku o bardzo szerokim rozprzestrzenieniu opisywanego poziomu tufitowego obejmującego przedpole Karpat polskich, Ukrainę, Ruś Zakarpacką i Rumunię. POLYCYCLIC TUFFITE HORIZON IN THE MIOCENE IN VICINITIES OF GLIWICE In the vicinities of Gliwice, western part of the Upper Silesian Coal Basin, a set of intercalations of pyroclastic rocks, forming a characteristic, polycyclic tuffite horizon, was found in the Miocene section at Ochojec. The stratigraphic analysis of the section showed that the tuffiles occurring in upper part of the Spirialis Clays, about 46 m above the top of the Gypsum Horizon, are of the Badenian age(lower part of the Upper Badenian). The Spirialis Clays yield an assemblage of planktonic microfossils with Spirialis and Globigerina (assemblage III A), characterized by quantitative predominance of pteropods in their lower part and of foraminifers in the upper (Fig. l). The deposits represent equivalents of the Chodenice Beds, described from the vicinities of Wieliczka and Bochnia (S. Alexandrowicz, 1961). In the Gliwice area, the Spirialis Clay are overlain by clays with rich assemblage of benthic foraminifers (assemblage III B), corresponding to the Grabowiec Beds (Fig. 1).The polycyclic tuffite horizon from Ochojec is 15 m thick, including 3.1 m of pyroclastic rocks.In the section, there occur 7 tuffite layers varying in thickness, here marked with symbol s TI – TVII(Fig. 1).Granulometric analysis showed changes in size of pyroclastic material not only within individual tuffite layers but also from the layer TIII upwards. The latter changes are connected with gradual decrease in size of the material at the base of successive layers. Microscopic studies carried out on powdered samples showed decrease in share of feldspars, biotite and heavy minerals at the advantage of pumice glass from the layer TIII to TVII (Fig. 2, Table I).Chemical analyses of separated volcanic glass, carried out by the classic and atomic absorption methods, showed rhyolite nature of glass present in the studied tuffites. The glass is, however, somewhat poorer in Al2O3 and CaO than rhyolites. The content of SiO2 in volcanic glass was found to increase from tuffite layer TIII to TVII; the exception is here sample no. P-II from tuffite layer TVI (Table 2). X-ray analyses of the fraction below 0.01 mm, separated from tuffites, showed increase in content of calcium montmorillonite from the layer TI to TVII (Fig. 3, Table 3), which may be explained by progressive increase of content of finer pumice glass.The results of mineralogical and petrographic studies on the tuffites indicate that successive stages of deposition of pyroclastic material were separated by times of deposition of clay material. The first stage of formation of the tuffite complex is represented by tuffites TI and TII the former of which is somewhat more basic than the latter. Tuffite TIII yields glass with the lowermost content of silica and, at the same time, pyroclastic material with the highest content of feldspars, biotite and heavy minerals.Tuffites TIV and TV may be regarded as related to TIII and, with reference to granulation and mineral and chemical composition of glass, they may be treated as an evidence for decreasingly acid character of volcanic source. The layers TIII­–TV represent the products of the main phase of eruption whereas the layer TVI appears related to eruption more basic in character, evidencing some trend to wanning of the volcanic source. The uppermost tuffite layer, TVII corresponds to a weak eruption and it marks ceasing of volcanic activity responsible for the origin of the whole polycyclic tuffite horizon.An establishment of detailed stratigraphic position of tuffites from Ochojec and of their petrographic nature makes it possible to compare this tuffite horizon with other Miocene pyroclastic rocks.Analogous tuffites are known from several sections of the Upper Badenian in the Upper Silesian Coal Basin, especially in the vicinities of Rybnik, Knurów, Gliwice and Kędzierzyn (S. W. Alexandrowicz, 1963). The studied tuffites may be also compared with those from top part of the Chodenice Beds in the vicinities of Wieliczka and Bochnia (W. Parachoniak, 1954; S. Alexandrowicz, 1961) and tuffite horizon from the vicinities of Mielec and Przemyśl (W. Parachoniak, 1962). The horizon is also known from the forefield of the Eastern Carpathians in Ukraine, e.g. from the Verbovec Beds and the base of the Pistyn Conglomerate (O.S. Wiałow, 1965; M. Kamieński, 1936). Other occurrences of that horizon may be traced in the forefield of the Eastern Carpathians in Roumania (N. Oncescu, 1959) and in the Transsylvanian Bassin, at inner side of the Carpathian arch (N. Oncescu. 1959; I. Cicha et al., 1975), where these deposits also occur in the Upper Badenian, i.e. Kosovian. In the parts of U.S.S.R., situated behind the Carpathians, analogous tuffite horizon is known to occur in the Banskie Beds (I.V. Venglinskij, 1975). The eastern Slovakia is also situated within the extent of this horizon.A very wide distribution and constant stratigraphic position of the tuffites implicates that it would be reasonable to differentiate it as lithostratigraphic unit with features of a correlative horizon and to name it as “Bochnia tuffite”, selecting the locality from Chełm upon the Raba Rivers (W. W. Parachoniak, 1954; E. Łuczkowska, 1955; S. Alexandrowicz, 1961) as the type locality.